I'm neither a Master nor a Mistress, so feel free to ignore me...

But seriously, are you engaging in bdsm play with her because *you* want to or because you think *she* wants you to? There is a *big* difference, and I know that I, as a sub, can tell the difference between when a partner is doing what he wants to do to/with me, and when he's just going through the motions, because he thinks that turns me on. Let me tell you...just doing it to please her isn't going to work.
I can tell you honestly that if I don't feel "it"...that sense of ownership/demand/control from a partner...then no amount of erotic play can really turn me on. A whipping without the intent behind it is just that...it might affect my body, but it doesn't touch my mind, doesn't take me out of myself the way I know it can.

I don't know if this makes sense, but that's just my take on the subject. Good luck to you with your girlfriend!